The British and French both had established colonies in the north so why did they never turn their attention south and colonise south america before the Spanish (or at least go to war with them for the colonies)?
The simple version is: Spain and Portugal were in America first. The Portuguese started expanding in the 14th century, and Spain started a little later. Both countries had two advantages: they are located in the Iberian Peninsula, the "end" of Europe; and they both had central strong unified governments before much of Europe, since they fought against the muslim caliphates in the Peninsula.
In 1494, both countries signed the Treaty of Tordesilhas, which divided all newly discovered land west of Europe between them - it created a meridian 370 leagues west of the Cape Verde Islands. Everything west of the Meridian should be spanish and everything to the east, portuguese. The Treaty was eventually forgotten, but it shows how far ahead of the other european countries they were at the time and it also explains why Spain got to colonize much of America.
Also, you should consider that the purpose of colonization was basically to make money for the crowns. The easiest way was to find gold and other precious minerals (which Spain found in the Aztec and Inca Empires), but also finding rare spices, raw materials and crops that could not be produced in Europe, like sugarcane. That's why tropical lands were favored by colonial powers, instead of temperate climates that were very similar to Europe. So Portugal and Spain, being the first ones in America, favored the colonization of Central and South America, leaving "european" North America to whoever came later.
France and the Netherlands were next in line. Both had some success in the Caribbean, colonizing many islands. Britain would come later, and at first british colonization was not sponsored by the crown, but actually an effort of religious refugees. So at first, british colonization was about settling and living in these lands and not an economic effort. The british crown would eventually get some tropical colonies, mainly in the Caribbean, through warfare, treaties and purchases. But North American colonization was still mainly about settling and estabilishing military outposts, except for the southern United States.
British colonization of the North came after after many of Spains conquests in Central/South America.
The French made some efforts before the British, but even then the Spanish led the way.
What years are you referring to where your stated situation was in place?